I don't have one to check, but if the menu has display options like front only, rear only, and F+R then try different settings with that. Also check the rear cam cable and connections. You could have a bad cable or a bad rear cam. Also check that you're on the latest FW. Rexing does have fairly decent factory support for warranty, less so for tech help, but you should be able to get this sorted out one way or another.
Phil